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Cash and Investments in Marketable Securities - Portfolio of Cash and Investments in Marketable Securities (Detail) - USD ($)
$ in Thousands
Dec. 31, 2016
Dec. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale investments $ 330,713 $ 250,444
Cash 4,355 1,842
Total cash and investments in marketable securities 389,102 308,944
Corporate Commercial Paper [Member] | Fair Value Hierarchy Level 2 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale investments 160,920 61,150
Corporate Notes and Bonds [Member] | Fair Value Hierarchy Level 2 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale investments 156,044 181,969
Obligations of U.S. government agencies [Member] | Fair Value Hierarchy Level 2 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale investments 13,749 7,325
Money Market Funds [Member] | Fair Value Hierarchy Level 1 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments in marketable securities 51,104 53,728
Certificates of Deposit [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments in marketable securities $ 2,930 $ 2,930